MR can effectively dispel the power of the Casket.
If an army has a unit with MR2, then that army could use those 2 dice supplied by the unit's MR to dispel the Casket's power, provided that the unit could 'see' the Casket following normal LOS restrictions.
If successfully dispelled in this manner, the Casket will cease to work on any unit that turn, not only the unit with MR.
Keep in mind that the Casket acts much like a normal bound spell, just with a variable power level. Magic Resistance acts just like having extra dispel dice, but only when that particular unit is threatened by magic. The extra 'dispel dice' generated by the MR will work just as well as normal dispel dice for canceling out bound or cast spells.